1. Field of the Invention
The present invention relates to a nozzle plate which is attached to a bottom of a container such as a ladle or tundish that accommodates molten steel and mounted on a sliding nozzle apparatus that controls a pouring rate of molten steel or the like, and more particularly, to a sliding nozzle plate to control a pouring rate of molten steel or the like discharged from the nozzle apparatus.
2. Description of Related Art
A sliding nozzle apparatus (hereinafter, also referred to simply as a “nozzle apparatus”) is attached to a ladle which receives molten steel discharged from a steel furnace such as a converter to carry, and pours the steel into a mold, or attached to a tundish which receives molten steel from a ladle and pours the molten steel into a mold, and is used widely as a pouring rate adjustment apparatus.
FIG. 6 shows a sliding nozzle apparatus generally used. The nozzle apparatus 141 is comprised of two plates, a fixed plate 121 that engages in a metal frame 153 provided on the bottom of a ladle, and a sliding plate 123 which is in pressure-contact with the lower surface of the fixed plate and is engaged in a metal frame 155 slidably. Hereinafter, the fixed plate and sliding plate are collectively referred to as a slide plate.
In order to prevent molten steel from leaking from a pressure-contact surface between the fixed plate 121 and sliding plate 123, the plates 121 and 123 are provided with a surface pressure mechanism (not shown) which applies a surface pressure in the longitudinal direction from the outside of metal frames 153 and 155. The fixed plate is engaged in the metal frame in a position such that a nozzle hole 3 is aligned with a nozzle hole 171 of an upper nozzle 143 disposed on the bottom of the ladle or the like. The sliding plate 123 is provided with a nozzle hole 5 corresponding to the fixed nozzle hole 3, and is slid to adjust an opening degree of the nozzle holes. The metal frame in which the sliding plate is engaged is coupled to the nozzle apparatus, for example, in a pin joint 153 at its end portion, and is slid by a hydraulic cylinder or the like in remote control through an operation rod 159.
The leak of molten steel occurs when respective nozzle holes of the fixed plate and sliding plate are in partly-open positions, while occurring hardly in full-closed positions. Only required are the function that controls a passage flow rate of the molten steel in the half-open positions and the function that simply stops the flow of the molten steel in the full-closed positions. In the partly-open positions, erosion is severe at portions such that the molten steel flow collides with the plate and that the molten steel flow changes its flow direction. Therefore, the fixed plate and sliding plate of the sliding nozzle apparatus have been handled as consumables.
The fixed plate and sliding plates are manufactured using expensive refractory materials, and are improved in shape and structure. For example, as shown in FIG. 12, Japanese Patent No. 3247941 describes an example of the nozzle plate reached from usage examples at portions where erosion is severe in consideration of a ratio of (g-f)/f based on the experiments on the plate. The document as described above disclose a decagon plate for a sliding nozzle provided with a dimension “g” substantially 1.5 times the diameter “f” of a nozzle hole and a dimension “h” substantially three times the diameter “f” of the nozzle hole in the longitudinal direction from the center position “Z” of the nozzle hole.
In the invention of Patent No. 3247941 as described above, since the dimension “g” is substantially 1.5 times the diameter “f” of the nozzle hole, it is understood that the plate 201 for a sliding nozzle has intense erosion and cracks in the nozzle hole in the longitudinal direction and has problems in durability.
FIG. 13 shows a schematic front view of a fixed plate 221 and sliding plate 223 for a sliding nozzle in the longitudinal direction in full open position. Arrows in FIG. 13 indicate pressure-contact directions 229 of the surface pressure mechanism. When the sliding plate is slid from the closed position, a distance “i” is increased between the end surface of the fixed plate 221 and the end surface of the sliding plate 223. At this point, the surface pressure mechanism acts in a portion corresponding to the distance i, but in the portion the sliding plate 223 is not positioned to be in contact. On the other hand, the opposite side (right side as viewed in FIG. 13) of the sliding plate 223 projects from the end of the fixed plate on the upper side, but the surface pressure does not act in this position. This is because the surface pressure mechanism not shown acts on the metal frames 153 and 155, but does not act directly inside of the metal frames.
Therefore, there occurs a deviation of the pressure-contact force of the surface pressure mechanism, and a tilt appears between the fixed plate 221 and sliding plate 223 as shown in FIG. 13. Hence, a gap 225 develops. The gap 25 is maximum when the displacement becomes maximum between the fixed plate 221 and sliding plate 223, i.e. the nozzle holes are full open.
FIG. 14 shows a schematic view of the fixed plate 221 and sliding plate 223 in the transverse direction when the nozzle holes are full open. Arrows in FIG. 14 indicate pressure-contact directions 229 of the surface pressure mechanism. The fixed plate 221 and sliding plate 223 are brought into intimate contact with each other by the pressure contact force of the surface pressure mechanism. The surface pressure mechanism applies the pressure outside the fixed plate 221 and sliding plate 223, the fixed plate 221 and sliding plate 223 thereby arch corresponding to the dimension of width, and therefore a gap 231 develops.
The gaps 225 and 231 have significant effects during casting. For example, during casting of molten steel, air is entangled to promote oxidization of the periphery of the nozzle hole of the plate, thereby causing fierce damage and resulting extremely reduced life.
Cracks generated on the periphery of the nozzle hole will be described below with reference to FIG. 15. As shown in FIG. 15, the nozzle plate is pressed against pressing metal 209 due to thermal expansion of the nozzle plate. For example, when segments of the nozzle plate are formed in the shape of a regular octagon as shown in FIG. 15, a pressing force 207 due to the pressing metal 209 acts toward the center of the nozzle hole 203 as shown by the arrows. Thus, the pressing force 207 gradually causes cracks 205 to occur around the periphery of the nozzle hole 203 having a relatively low strength.
For example, as shown in FIG. 15, the cracks 205 develop in the shape of a cross, and propagate and extend in the plate for a sliding nozzle. When such cracks 205 occur, for example, air is entangled, and oxidization is promoted on the periphery of the nozzle hole of the plate, thereby causing fierce damage and resulting extremely reduced life.